1. Field of the Invention
This invention relates to the oxidation of liquid aqueous ferrous chloride solution to obtain a liquid aqueous ferric chloride solution utilizing a promoter cation. Particularly the invention relates to the recovery of the chlorine content of waste hydrochloric acid (HCl) steel mill pickle liquor--WPL--as hydrochloric acid. Also the invention relates to the decomposition of aqueous ferric chloride solution to hydrogen chloride gas and waste vapor, at subatmospheric pressure.
2. Description of the Prior Art
Hydrochloric acid is used to pickle steel to remove the oxide coating from the base metal. Acid of 20-30% concentration is generally used as makeup for the picklers, with a preference for the higher strength acids. The pickling operation produces a waste material known as "waste HCl pickle liquor" and more commonly spoken of as "WPL". Typically WPL contains in addition to water, about 13-25 weight percent of ferrous chloride (FeCl.sub.2), less than 1 weight percent of ferric chloride (FeCl.sub.3), variable but small amounts of free HCl, normally about 1% or less, and small amounts of organic inhibitors which decrease the attack of the acid on the base metal.
WPL is an acidic material and its disposal, when environmental pollution is to be avoided, is a serious economic problem. Commonly the WPL is neutralized with lime; the slurry dewatered; and the solid used as land fill. It can be seen that this is an expensive operation, even when a dumping area is readily available.
Hydrochloric acid is relatively expensive and the recovery of the chlorine content of the WPL, as hydrochloric acid, has been a goal of picklers from the beginning of the extensive use of hydrochloric acid as a pickling acid. The processes offered commercially today all involve thermal decomposition of the WPL and recovery of the HCl gas content of the gaseous decomposition products by water scrubbing. Illustrative of a recently developed process now being offered is: WPL is sprayed through a burner area onto a bed of refractory material, where it decomposes into HCl gas and iron oxide. The iron oxide particles are periodically removed by probing and vibrating the bed. The gases are scrubbed to recover 20.degree. Be acid. The system is closed-loop to assure minimum environmental contamination (C&E News, p. 32, May 18, 1970).
Needless to say, the roasting processes--the only ones now being offered--are very expensive, both in investment costs and in direct operating costs.
The concentration on roasting processes for treatment of WPL is the result of the lack of success of the art in oxidizing ferrous chloride in aqueous solution to ferric chloride. Crummett et al., U.S. Pat. No. 2,677,598, issued May 4, 1954, demonstrates that liquid aqueous ferrous chloride solutions when contacted with free oxygen produce ferric chloride only to a slight extent; also that the presence of hydrochloric acid retards the reaction. Crummett provides a process involving the addition of hydrochloric acid to the aqueous ferrous chloride solution to a 3-5 N range; then oxidizing the HCl-FeCl.sub.2 solution in the presence of a large amount of the chloride salt of an anion exchange resin--a preformed salt.
Crummett et al postulate that an equilibrium exists in aqueous solution essentially favoring only the ferrous ion. They discovered that the ferric chloride was chemisorbed from aqueous solution by the solid salt of an anion exchange resin and HCl; thus, the equilibrium was shifted enough to permit substantial conversion of the ferrous ion to ferric ion. The chemisorbed ferric chloride and hydrogen chloride is desorbed with water, or preferably with dilute hydrochloric acid to obtain a solution of ferric chloride containing a large amount of dissolved HCl.
Hudson U.S. Pat. No. 2,428,221, issued Sept. 30, 1947, shows air treating of WPL, which has been fortified with added hydrochloric acid, to convert the ferrous ion to ferric ion. Enough HCl is added to obtain a stoichiometric reaction between ferrous chloride and HCl. Hudson himself points out the economic disadvantages of this operation.